1. Field of the Invention
This invention relates to electrical circuits, and in particular to a circuit for the input stage of an automotive ignition control circuit.
2. Prior Art
Until relatively recently the spark for igniting the fuel air mixture in the cylinders of internal combustion engines has been generated by electromechanical means, typically, a distributor rotor and cam which mechanically opened and closed electrical contacts to control a coil. The coil supplies sparks to the various cylinders in the proper sequence and for the proper duration. It is now well known, however, that performing this function electronically provides substantial advantages over the prior mechanical techniques. Unfortunately many of the electronic techniques for performing this function waste power by charging the coil for an excessively long period. In response to this concern, techniques have been developed for generating a control signal having the minimum on-time necessary to charge the coil.
In one prior art technique, now embodied in Fairchild Camera and Instrument Company product 7357, the minimum on-time necessary to charge the coil is generated by using a magnetic pick-up in the distributor which mechanically or electromechanically generates a nonlinear triangular waveform. Unfortunately such mechanical or electromechanical devices are expensive and require careful adjustment.